以下是think 的两个用法，不要混淆。

1. think that ...宾语从句：宾语从句陈述一个主谓事件。

2. think + 宾语 + 形容词：认为宾语怎么样。形容词作宾补。

就本句而言，两种用法都可以，第2句相对简练：

I have to walk to school, because my parents think it is good for health.

I have to walk to school, because my parents think it good for health.